As is known in the art, there is a need for high-sensitivity, small, linear power sensors without the use of complex, large circuits and external biasing. Sensors that are highly integrated with power amplifiers, for example, are useful at the input, output, and inter-stage networks to detect any inefficiency in the amplifier performance. These types of integrated power sensors may be used in a system that is tunable for optimal performance. Broad bandwidth is also a desirable feature for any system to detect spurious signals. The integrated microstrip-based power sensor with direct contact thermopiles solves these problems with small size, linearity, no external biasing, broadband operation (2-18 GHz tested), and improved sensitivity.
Field effect transistor (FET) mesas may be used to detect power passively without external bias. Planar transmission lines, such as microstrip, dissipate power in the form of heat as they propagate. This dissipated power is related to the insertion loss per unit length of the microstrip line given an initial input power value. If a heat absorbing material is placed in close proximity to the transmission line, the dissipated power will heat the material. By isolating the heat absorbing material so that it is cool furthest from the RF transmission line, a temperature differential builds across it. Certain material combinations, called thermocouples, respond to this thermal gradient with a detectable voltage gradient and convey information about the input power from the RF microstrip line. It is based on the Seebeck effect (measured in volts per degree C.) in which a voltage appears between two dissimilar materials if a temperature gradient exists between two junctions along them. This approach is highly integrated and does not require couplers. Lossy transmission line is the source of ohmic loss, or heat. These thermocouple fingers are connected in series to create a thermopile with increased sensitivity. The thermopiles are placed on both sides of the transmission line and the total sensitivity is equal to the total voltage detected over the power dissipated in V/W. Increased sensitivity can be obtained with improved thermal isolation of the hot junction by micromachining or locally etching the bulk GaAs substrate under the center conductor and hot junctions. Other parameters employed to increase sensitivity are: the number of thermopiles, thermopile length, thermopile width, thermopile pitch, and proximity to heat source.
As shown in the equations below, the sum of the temperature differentials (Ti, To) between the hot and cold junctions for a series of thermocouples is multiplied by the Seebeck coefficient (αk) to yield a detected voltage (Vout) for the thermopile. The sensitivity (S) is equal to the detected voltage divided over the power dissipated.Seebeck, αtc˜300 μV/C
      V    out    =            α      tc        ⁢                  ∑                  i          =          1                N            ⁢              (                              T            i                    -                      T            o                          )            Sensitivity, S=Vout/Pdiss, (V/W)
To determine the dissipated power, electromagnetic simulations such as Momentum and HFSS are used to determine the insertion loss as a function of frequency, and then the dissipated power is calculated as follows. For example, following the equations below, if an input power of 2 W is applied to a line with 0.5 dB of insertion loss, the dissipated power level will be 0.22 W.InsertionLoss(dB)=10 log PinPoutPout=Pin/(10(InsertionLoss/10))Pdissipated=Pin−PoutPdissipated=Pin−Pin/(10(InsertionLoss/10))
Reference is made to the following articles: “Broadband thermoelectric microwave power sensors using GaAs foundry process” by Dehe, A.; Fricke-Neuderth, K.; Krozer, V.; “Microwave Symposium Digest, 2002 IEEE ”MTT-S International, Volume: 3, 2002 Page(s): 1829-1832; “Free-standing Al0.30Ga0.70As thermopile infrared sensor”, by Dehe, A.; Hartnagel, H. L.; Device Research Conference, 1995. Digest. 1995 53rd Annual, 19-21 Jun. 1995 Page(s): 120-12; and “High-sensitivity microwave power sensor for GaAs-MMIC implementation” by Dehe, A.; Krozer, V.; Chen, B.; Hartnagel, H. L.; Electronics Letters, Volume: 32 Issue: 23, 7 Nov. 1996 Page(s): 2149-215.
One such type of power sensor is described in an article by A. Dehe et al., entitled “GaAs Monolithic Integrated Microwave Power Sensor in Coplanar Waveguide Technology” published in the IEEE 1996 Microwave and Milli-meter Wave Monolithic Circuits Symposium, pages 179-181. In such article, the authors show a power senor having sensitivity levels of 0.55 V/W with coplanar waveguide using AlGaAs mesa for a terminated load of 50 Ohms. In some applications, it would be desirable to obtain a power sensor having a higher level of sensitivity.